This invention relates generally to pulse width modulated DC to DC power converters and in particular to a DC to DC power converter having an additional control loop for optimized dynamic control of such a converter when operating in a constant load current regulation mode or an output voltage regulation mode.
Pulse width modulator power converters have many types of control techniques, among the most popular forms being conventional voltage mode control, and current mode control. With voltage mode control, a control voltage is compared to a constant ramp to determine the appropriate duty cycle. Current mode control sums a sample of a filter inductor's current waveform with an external fixed-slope ramp signal.
Current mode control of switching regulators is well known in the art where an inner or secondary loop is used to directly control peak inductor current with an error signal rather than directly controlling the duty ratio of a pulse width modulator as in conventional voltage mode switching regulator converters. Hence, instead of comparing the error voltage to a voltage ramp, it is compared to an analogue of the inductor current forcing the peak current to follow the error voltage. (See Application Handbook 1985-86 Note by Unitrode Corporation of Lexington, Mass. "A New Integrated Circuit For Current-Mode Control," pp 210-218, 1983).
In a power supply for a traveling wave tube focusing coil of a radar system, a need arises to provide a constant current to the focusing coil. However, due to variations in focusing coil parameters because of different manufacturers, due to different operating current levels for coils from the same manufacturer, and also due to thermal heating of the focusing coil, it is necessary to support various output voltages V.sub.o of a DC to DC converter in order to maintain a constant current output through various loads. The DC to DC converter dynamic operation is optimal for only one value of the output voltage.
A Buck regulator DC--DC converter for driving traveling wave tube focusing coils is described in a paper entitled "State Space Analyses of Buck Regulated DC--DC Converters for Inductive Focusing Coils," by C. P. Schultz, Proceedings of Power Conversion, Oct. 19, 1989, pp. 502-510. A current mode control type of converter circuit is described having an inner current feedback loop inside the primary regulation loop. A stabilizing voltage ramp may be injected into the regulation loop to maintain stability of the circuit for duty ratios greater than 0.5. When such a circuit is adapted to have the primary regulation loop sense output load current in order to maintain a constant output load current, the circuit has optimum dynamic performance for one voltage output; an example of optimum dynamic performance is minimized variations in output voltage due to variations of the input voltage. A circuit ratio (K) of primary interest is the slope of the injected voltage ramp referred to as (M) relative to the "falling slope of the inductor current" (referred to as M.sub.2) after being sensed by a current sensor. M.sub.2 for a Buck regulator topology equals V.sub.o /L. Hence, as the output voltage changes and the ramp slope remains fixed, this ratio varies resulting in non-optimal dynamic response for such a converter circuit. It can be seen from this ratio that if a ramp slope is generated that is proportional to V.sub.o, then this ratio would be a constant resulting in an optimally controlled converter especially for improving ripple rejection or minimizing the effects of noise injected into the inner current feedback loop.